Oceanographic measurements play an important role in all aspects of ocean science and engineering. Typical applications in applied marine science and engineering include mapping of pollutant transport, environmental monitoring, search for pollution sources, national security, target localization, search and rescue, marine geophysics, resource exploration and resource production. In basic marine science, oceanographic measurements are a key component, together with modeling, of the development of the fundamental understanding which subsequently provides the basis for engineering, management, and policy solutions.
A common problem for all these applications of marine measurements is the fact that they have become extremely platform limited. The performance/cost ratio has grown exponentially for the computer technology used for modeling and analysis, but has been diminishing or, at best, remained unchanged in terms of the platforms. Most oceanographic measurements are still performed from ships.
The development of inexpensive autonomous underwater vehicle technology has provided a breakthrough in terms of performance/cost ratio, with the potential for revolutionizing the area of experimental oceanography in deep as well as shallow water. The concept of autonomous oceanographic sampling networks (AOSN) provides a particularly powerful technology. Autonomous oceanographic sampling networks are described, for example, by T. B. Curtin et al, in "Autonomous Oceanographic Sampling Networks ", Oceanography, Vol. 6, No. 3, 1993, pages 86-93. However, the AOSN technology has its inherent limitations because of its particular functionality.
A fundamental problem facing oceanographic measurement techniques is a tradeoff between coverage and resolution. A measurement system may cover a large area, but only be able to produce results over a very coarse grid in space and time. This is the case for ship-based collection of water samples. Other measurement techniques are highly accurate and capable of measuring small spatial and temporal variations, but cover only a very small volume. In general, the common methods applied in oceanography provide a compromise between the two extremes. While a wide variety of oceanographic measurement techniques exist, none is capable of simultaneously providing wide area coverage of the entire water column and high resolution.
One widely-used oceanographic measurement technique is acoustic tomography, which is a spin-off of the acoustic methods developed for anti-submarine warfare. Acoustic tomography uses the fact that sound speed is strongly dependent on temperature, salinity and pressure, properties which can therefore be inferred in an ocean volume by analysis of acoustic transmissions within the source-receiver network deployed in the volume. Typically, an acoustic tomography network includes five or six vertical receiver arrays spanning the water column. Each array has an acoustic source transmitting a coded sequence to the other arrays. The analysis is performed by simulating the received signals using a propagation model, the input parameters of which are varied to reproduce the measured data. Acoustic tomography provides coverage in the vertical slices between the sources and receivers, but virtually no information about regions between the slices. The spatial resolution of acoustic tomography is very limited, typically on the order of 100 meters in the horizontal direction and 10 meters in depth. Furthermore, the vertical arrays have fixed positions. The main advantage is the coverage in terms of frequent snapshots, allowing for tracking of oceanographic dynamics. Acoustic tomography is described, for example, by P. F. Worcester, et al. in "A Review of Ocean Acoustic Tomography: 1987-1990", Reviews of Geophysics, Supplement, pages 557-570, April 1991 and by W. H. Munk, et al. in "Ocean Acoustic Tomography: A Scheme for Large Scale Monitoring", Deep Sea Research, Vol. 26A, pages 123-161, 1979.
One implementation of the autonomous ocean sampling network employs moored buoys which function as network nodes for a long-term multiple underwater vehicle presence in the ocean. The objective is to provide an economically feasible capability for a repeated synoptic characterization of large scale oceanographic phenomenon. The key to such a system is a small, low-cost autonomous underwater vehicle which can be operated reliably over extended, unattended deployments at sea. The vehicles traverse the network recording temperature, salinity, velocity and other data, relaying key observations to the network nodes in real time and transferring more complete data sets after docking at a node. Each network node consists of a base buoy or mooring containing an acoustic beacon, an acoustic modem, point sensors, an energy source and one or more autonomous underwater vehicle (AUV) docks. The motivation for multiple vehicle surveys is that the quality and utility of the data obtained improves much faster than the number of vehicles for large scale dynamic ocean phenomenon.
Arrays of hydrophones are widely used. Multiple hydrophones may be spaced along a cable towed behind a ship to form a towed array. The hydrophones may be used in seismic exploration to sense seismo-acoustic energy reflected from structures below the ocean floor or may be used to localize an acoustic source, such as a submarine. Towed arrays have fixed configurations and may be difficult to maneuver. Fixed arrays of hydrophones may be used for similar applications, but lack flexibility and maneuverability.
Existing oceanographic measurement techniques have one or more drawbacks, including low resolution, lack of mobility, lack of flexibility, small acoustic apertures and an inability to adapt to changing conditions it is desirable to provide oceanographic measurement apparatus and methods which overcome one or more of these drawbacks. In particular, it is desirable to provide oceanographic measurement apparatus and methods which permit simultaneous measurement of target characteristics from different directions, which have mobility and flexibility and which permit large acoustic apertures to be implemented at low frequencies.
According to a first aspect of the invention, an oceanographic sampling system comprises a plurality of underwater vehicles disposed in an array having an array configuration and array control means for controlling the array of underwater vehicles as data is acquired. Each underwater vehicle includes a propulsion system for moving the underwater vehicle independently of the other underwater vehicles, a sensor for sensing an ocean parameter and providing sensor data representative of the ocean parameter as the underwater vehicle moves, a navigation subsystem for determining position data representative of the position of the underwater vehicle as the sensor data is acquired and a synchronizing subsystem for time synchronizing the sensor data and the position data acquired by the underwater vehicle with sensor data and position data acquired by the other underwater vehicles. The underwater vehicles function as a phased array, and phased array analysis techniques may be applied to the time-synchronized sensor data and position data.
The array control means may control the array configuration, array movement and underwater vehicle function, alone or in combination. The array may be controlled in a preprogrammed manner or may be controlled adaptively in response to a condition that is determined during acquisitions of sensor data. The array control means may be located externally of the underwater vehicles or in one of the underwater vehicles, or may be distributed among the underwater vehicles. The array control means may include means for controlling the shape of the array, the size of the array, the depth of one or more of the underwater vehicles in the array, the direction of movement of the array, the function of one or more of the underwater vehicles in the array and/or the velocity of the array.
The synchronizing subsystem and the navigation subsystem may utilize optical communication channels between selected ones of the underwater vehicles. In a first embodiment, the optical communication channels each include light-emitting diodes and photosensors for optical communication. In a second embodiment, the optical communication channels each include lasers and photosensors for optical communication. The synchronizing subsystem and the navigation subsystem may alternatively use high frequency acoustic communication channels between selected ones of the underwater vehicles. One or more of the underwater vehicles may include an acoustic excitation source.
According to another aspect of the invention, a method for oceanographic sampling is provided. Two or more underwater vehicles are disposed in an array. Each underwater vehicle includes a propulsion system for moving the underwater vehicle independently of other ones of the underwater vehicles. An ocean parameter is sensed with each of the underwater vehicles and sensor data representative of the ocean parameter is provided as the underwater vehicle moves. Position data representative of the position of each of the underwater vehicles is determined as the sensor data is acquired. The sensor data and the position data acquired by each of the underwater vehicles is time synchronized with sensor data and position data acquired by other ones of the underwater vehicles. The array of underwater vehicles is controlled as the time-synchronized sensor data and position data are acquired, The array of underwater vehicles thereby functions as a phased array.